1. Field of the Invention
The present invention relates to a linear amplifier for amplifying a composite signal of plural frequency components. More particularly, the invention relates to an amplifier having a superior linearity without lowering an electric power load efficiency.
2. Description of the Related Prior Art
In general, a voltage waveform of an output signal of an amplifier has a harmonic distortion component.
A 2nd-order distortion component and a 3rd-order distortion component in the harmonic distortion component can not be ignored because of their amplitudes in comparison with a 1st-order component. The distortion components, the central frequency of which is a frequency of an input signal, are generally removed by a filter.
However, the 3rd-order distortion component, and more specifically, a 3rd-order intermodulation distortion component correlated to frequencies F1 and F2 is close to the frequency of the input signal, and therefore, it can not be removed by the filter, so that it is required to reduce the occurrence level of the 3rd-order distortion component to a lower level.
A method for removing this 3rd-order intermodulation distortion component was discussed in the article titled "A New Method of Third-Order Intermodulation Reduction in Nonlinear Microwave", pages 245 through 250 of IEEE [Transactions on Microwave Theory and Technics, Volume MTT 34, No.2] published in February 1986 .
According to the principle described in the article, the 3rd-order intermodulation distortion component can be removed by extracting the (F1-F2) component from the output of the amplifier and feed-backing it to the input side when the input signal has two frequency components F1 and F2. However, the device described in the abovementioned article is a feedback type distortion compensating amplifier.
As electric power is fed-back in the feedback type amplifier, the transmission gain is reduced by the feed-back amount. A feed-forward type amplifier may be used to solve this problem.
FIG. 8 shows an example of a circuit for compensating the distortion of a signal of the amplifier having the feed-forward type structure. FIG. 8 shows an example of a circuit for compensating distortion component D in the case where distortion component D is added in an amplifier 81 for amplifying the input e.sub.i.
The system for compensating the distortion component shown in FIG. 8 constitutes the feed-forward type distortion compensating circuit. In FIG. 8, numeral "82" is an adder, in which distortion component D is added. "83" is a circuit having a coefficient K.
"84" is an adder for adding the input e.sub.i to the output of circuit 83 having coefficient K.
"85" is an auxiliary amplifier which has almost the same gain as main amplifier 81 and which amplifies the output of the adder 84. "86" is an adder for adding the output of the auxiliary amplifier 85 to the output of the main amplifier 81, added with distortion component D.
In the circuit of FIG. 8, when the output of adder 86 is e.sub.o, the distortion D is compensated as shown in the following equation; EQU e.sub.o =(A1+A2+A1A2k)e.sub.i +(1+kA2)D,
where, A1 and A2 are gains of the amplifier 81 and the amplifier 85, respectively. Further, when kA1 is equal to kA2 equal to -1, e.sub.o becomes equal to Ale.sub.i, so that the distortion D is deleted.
Hereupon, the auxiliary amplifier 85 having almost the same gain as the main amplifier 81 is required for removing the distortion component D in the conventional circuit as shown in FIG. 8. Further, it is also required in order to maintain better efficiency to sufficiently improve the stability of each loop.
The distortion component generated in the auxiliary amplifier 85 is not reduced and the whole characteristic of the distortion is determined by the characteristic of the auxiliary amplifier 85.
Further, the efficiency of the amplifier becomes lower because the compensating circuits such as circuit 83 having coefficient k and an auxiliary amplifier are provided.